1. Field of the Invention
The present invention relates to a temperature detection circuit, and more specifically, to a temperature detection circuit capable of high-accuracy temperature detection under low power voltage conditions.
2. Description of the Related Art
Recently, there is increasing demand for portable devices, such as mobile phones, that are capable of low voltage operation with low power consumption. For such devices, it is extremely important to have a high-accuracy temperature detection circuit capable of operating under such low power voltage conditions.
FIG. 1 is a circuit diagram of a known temperature detection circuit. The temperature detection circuit includes a first voltage source circuit 101, which generates a voltage that is a linear function of absolute temperature, a second voltage source circuit 102, which generates a predetermined reference voltage that is not affected by the ambient temperature, and a subtraction circuit 103.
FIG. 2 is an example circuit for the first and second voltage source circuits 101 and 102. In FIG. 2, each reference in left-hand side represents a part with respect to the first voltage source circuit 101. Each reference in right-hand side represents the corresponding part with respect to the second voltage source circuit 102. The first voltage source circuit 101 includes two field effect transistors, M101a and M102a, and has a circuit configuration equal to the second voltage source circuit 102 excepting a ratio between a gate width W and a gate length L of the two field effect transistors, M101a and M102a. 
The field effect transistor M101a is a depletion-type field effect transistor (FET) having a gate containing an n-type impurity concentration. The field effect transistor M102a is an enhancement-type field effect transistor having a gate containing a p-type impurity concentration.
The first voltage source circuit 101 outputs a PTAT (proportional-to-absolute-temperature) voltage by adjusting a ratio between the channel lengths of the field effect transistors M101a and M102a. Similarly, the second voltage source circuit 102 outputs a predetermined voltage by adjusting a ratio between the channel lengths of the field effect transistors M101b and M102b. Since the PTAT voltage output from the first voltage source circuit 101 has a small temperature dependence, it is not possible to form a high-accuracy temperature detection circuit only with the first voltage source circuit 101. Accordingly, the temperature detection circuit includes the second voltage source circuit 102 (i.e., a reference voltage generator) and the subtraction amplifier 103 which subtracts a reference voltage output from the second voltage source circuit 102 from the PTAT voltage output from the first voltage source circuit 101 as shown in FIG. 1. With this circuit configuration, the temperature detection circuit can operate with high accuracy even under low voltage conditions.
As for the first voltage source circuit 101, an output voltage of the first voltage source circuit 101 is determined by a value of a formula,{Vth102−(β101/β102)½×Vth101}where Vth101 is threshold voltage, β101 is conductivity coefficient for the field effect transistors M101a, Vth102 is threshold voltage, and β102 is conductivity coefficient for the field effect transistors M101b. According to results of a number of experiments, it is found that the value of the formula has fluctuation in temperature coefficient (temperature dependence) and fluctuation in DC voltage due to variations occurring during manufacturing processes.
As for the second voltage source circuit 102, it is also found that the reference voltage has fluctuation in DC voltage due to variations occurring during manufacturing processes, and moreover the fluctuation in DC voltage is not small according to results of a number of experiments. Further, an input offset voltage is generated at each input terminal of the subtraction amplifier 103. The input offset voltage also has fluctuation in temperature coefficient and fluctuation in DC voltage due to variations occurring during manufacturing processes according to the results of a number of experiments similar to the value of the above-described formula {Vth102−(β101/β102)½×Vth101}.
An output voltage Vtemp of the subtraction amplifier 103, obtained by subtracting the reference voltage output from the second voltage source circuit 102 from the PTAT voltage output from the first voltage source circuit 101 is expressed by the following formula:Vtempk1×T+k2+σtemp1×T+σtemp2   (a)where T is temperature, K1 and K2 are coefficients determined by a subtraction rate and a gain, and σtemp1 and σtemp2 are coefficients determined by the multiplication factor of the subtraction amplifier 103, the fluctuation of the formula {Vth102−(β101/β102)½×Vth101}, and the input offset voltage.
Another known temperature detection circuit includes a first voltage source circuit to generate a first voltage by utilizing a work function difference between the gates of the two field effect transistors, a second voltage source circuit to generate a reference voltage by utilizing a work function difference between the gates of a plurality of field effect transistors, and a subtraction circuit which subtracts the reference voltage from the first voltage. However, the formula (a) indicates that the output voltage Vtemp includes both fluctuations in temperature coefficient and DC voltage. When the gain of the subtraction amplifier 103 increases, the fluctuations in the PTAT voltage, the reference voltage and the input offset voltage of the subtraction amplifier 103 also increase. Accordingly, it may not be possible to accurately detect an ambient temperature around the device under low voltage conditions with known temperature detection circuits.